1. Field of the Invention
This invention relates to a golf ball comprising a core and a cover. More particularly, it relates to a golf ball wherein a thermoplastic polyurethane elastomer comprised of an aliphatic diisocyanate and having specific viscoelasticity is used as the cover whereby the ball offers an improved initial velocity (or repulsion), an increased carry, a pleasant hitting feel, controllability, improved scuff resistance upon iron shots, little discoloring of cover surface, and good moldability.
2. Prior Art
With respect to the resin component of golf ball covers, ionomer resins in the form of ethylene/(meth)acrylic acid copolymers have heretofore been widely used and accepted as the cover stock for two-piece solid golf balls and some wound golf balls because of their hitting durability and cut resistance.
Ionomer resins of this type, however, generally have drawbacks including difficulty to impart spin as desired upon iron shots and poor controllability since they provide a hard hitting feel as compared with balata rubber conventionally used as the cover stock and have a higher hardness than balata rubber.
For an improvement in this regard, it was proposed (in U.S. Pat. No. 4,884,814 and JP-A 308577/1989) to use a soft/hard ionomer blend of a relatively soft ionomer resin in the form of an ethylene/(meth)acrylic acid/(meth)acrylate terpolymer with a certain amount of an ionomer resin in the form of an ethylene-(meth)acrylic acid copolymer having a specific spectrum of physical properties as a golf ball cover. It was also proposed (in JP-A 277208/1993) to use at least two metal salts of ethylene/unsaturated carboxylic acid/unsaturated carboxylate terpolymers having low flexural modulus as the cover.
These proposals are effective techniques for significantly improving the hard hitting feel and poor control which are problems of prior art golf balls using an ionomer resin in the form of an ethylene/(meth)acrylic acid copolymer as the cover.
The golf ball covers according to the above-referred proposals, however, suffer from the following problems. Namely, the fact that the cover is soft and receptive to spin upon iron shots indicates an increased frictional force between the club face and the cover. As a consequence, balls using a hard core like two-piece solid golf balls often give rise to the phenomenon that the ball surface is scuffed or scraped upon iron shots because the cover surface can be rubbed off by furrows in the iron club face.
Moreover, the ionomer resin covers experience a drastic drop of restitution as their hardness lowers. The covers themselves are reduced in restitution, leading to a substantial drop of restitution of balls as a whole.
Then, many attempts were proposed to use thermosetting polyurethane elastomers having a relatively low price, good feel and scuff resistance as a substitute for balata rubber or ionomer resin (U.S. Pat. Nos. 4,123,061, 3,989,568, and 5,334,673).
Although the above-proposed thermosetting polyurethane elastomers are improved in scuff resistance which is a problem of softened blends of ionomer resins, they require a complex operation of effecting curing reaction after injection of stock material, for example. More efforts must still be made to enable mass scale production. Since thermosetting polyurethane elastomers including only aliphatic isocyanate have a slow rate of curing reaction, it is preferred to partially use aromatic isocyanate to accelerate the reaction. The use of aromatic isocyanate, however, causes the cover to yellow with the lapse of time. Even when white enamel paint is coated to the outside for concealing, the ball cannot keep its outer color tone unchanged as the polyurethane cover yellows.
On the other hand, many investigations were made on the covers using thermoplastic polyurethane elastomers (U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282). Although these thermoplastic polyurethane elastomers are effective for improving scuff resistance upon iron shots and moldability, they are still unsatisfactory at present with respect to hitting feel, control, and initial velocity (repulsion). There is a desire to develop a golf ball having better performance and higher quality.